Inorganic scales that result from the deposition of solid salts from high brine supersaturated solutions injected into and/or removed from underground petroleum bearing formations frequently lead to lost production or well abandonment. Deposits can plug the formation near the wellbore, tubing string, downhole safety valves and other valves, and casing perforations. Subsurface pumps can stick and the operation of surface lines and equipment can be restricted.
Deposition can be initiated by a variety of factors including pressure, pH, temperature, turbulence, surface characteristics, or mixing of incompatible fluids. Incompatible fluids are frequently encountered during water flooding operations. A common factor that causes scale is pressure reduction encountered by fluids as they enter the wellbore during production. The partial pressure of CO.sub.2 decreases which can lead to the precipitation of CaCO.sub.3.
A variety of scales, both organic and inorganic, cause production problems. Common inorganic scales are calcium carbonate, calcium, barium, and strontium sulfate, and iron sulfides. The calcium salts are the most common.
Scales can either be removed or inhibited. Wells can also be re-perforated in order to circumvent the plugged area. Plugging is often seen at the perforation, where pressure changes are first seen. The well can also be fractured in order to bypass previously scaled areas. Both perforation and fracturing are expensive and only temporary remedies since scales will rapidly re-form.
The most efficient way of dealing with scale is to inhibit its formation. Chemicals can be sequestrants or work as a threshold inhibitor. Sequestrants form combination pairs with a species normally involved in precipitation, such as calcium ions. The interaction with the sequestrant is on a molar basis and therefore requires a large amount of chemical. While effective, this procedure could be cost limiting.
A much more effective chemical treatment is to use a threshold chemical, that is, one that inhibits at a concentration well below equimolar amounts. Threshold chemicals can be effective at concentrations as low as 1/1000th the concentration of the scaling mineral. Precipitation is a complicated process involving supersaturation, nucleation, and crystal growth. An inhibitor can function by one or more mechanisms. It can interfere with the nucleation process or rate. It can interfere with the growth process by blocking crystal growth sites. Crystal growth patterns can be altered. It can also prevent adhesion of crystals to one another or metal walls. In order to be effective, the scale inhibitor must be present during the nucleation stage of crystal growth.
The most common classes of inhibitor chemicals are inorganic phosphates, organophosphorus compounds and organic polymers. The common polyphosphates are sodium tripolyphosphate and hexametaphosphate. Organophosphorus compounds are phosphonic acid and phosphate ester salts. The organic polymers used are generally low molecular weight acrylic acid salts or modified polyacrylamides and copolymers thereof. Phosphonates and polymers are more thermally stable than polyphosphates or phosphate esters. Polyphosphates and phosphate esters hydrolyze at high temperatures forming orthophosphates, the metal salts of which may be more insoluble than the scales that they are intended to inhibit.
The polyphosphates have low brine solubilities and are therefore frequently injected as solids. They can be injected into the well bore by bypass feeders, baskets, filter packs, and bottomhole well packs. They can also be placed into the formation through fractures along with the fracturing fluids. The chemical then dissolves slowly, resulting in a steady, low concentration of inhibitor.
The phosphonates, phosphate esters and polymers are more water soluble and are therefore used as solutions. Either the wellbore or the formation can be treated. Both batch and continuous methods are used for treating the wellbore. They can also be added as a component of a fracturing fluid. These treatments are not optimum, since chemical does not contact the point of initial scale formation--the formation face or casing perforations. However, the tubing string and surface equipment will be treated.
A more efficient and less costly procedure is a "squeeze" treatment, in which the chemical is injected into formation. Production is halted while chemical is injected at a pressure below frac pressure. The chemical optimally penetrates the formation to a distance 1-6 feet radially from the wellbore. Inhibitor will then be released into the wellbore as production is resumed. Ideally, the concentration of inhibitor is constant and low (at a concentration slightly above that required for total inhibition--generally 2-4 ppm). The lifetime of a squeeze depends on the flow rate, oil/water ratio and many other factors but can last for 6 months and even up to 2 years.
The residual concentrations of phosphonates and phosphate esters can be easily and accurately determined in oil field brines by a titration method. However, no accurate method exists for field testing of polymer residuals. Many users are interested in a polymer squeeze but are reluctant to perform one due to the problem of residual determination.
Since precipitation squeezes are believed to be superior to adsorption squeezes, a superior squeeze chemical should be one whose calcium salt has a very low solubility. However, the solubility should not be so low that the concentration of produced inhibitor is below that required for effective scale inhibition.
Also, an inhibitor is needed that will inhibit barium sulfate scale as well as the more common scales. Barium sulfate scale is almost impossible to remove once formed and is becoming a more frequent problem, especially in Alaska and many foreign locations. In locations such as the North Sea, barium and strontium sulfate inhibition is becoming a major problem as waterflooding operations involving sea water increase.